Gmail marks messages as spam

I got campaign emails working - sort of. I did a test and was able to manually get all the messages out. I created a few dummy targets with email addresses that I control, two in Gmail and two on private domains. Those on private domains were delivered. Messages to the Gmail accounts got immediately flagged as spam and sent to the Gmail cloud spam folder before they even made it to the mail client.

Any suggestions? Or do Gmail users just never see our legitimate business correspondence?